The Phase of the Day: Something else the Gregorian Calendar Gets Right

In the last calendars post, we concluded that days and years were the proper periods for a calendar. But their underlying natural phenomena are cycles without beginning and without end. To be useful in a calendar, we need to be able to put particular events on particular days and years. We need to be able to count and label days and years. To do that, we need to pick a dividing line between one day and the next, between one year and the next. When does one day start and one day end? When does one year roll over to the next? We'll call these properties the phases of our days and years. For the phase of the day, we'll find that the Gregorian calendar once again gets something spot on.

The spinning of the Earth causes each day to cycle through two phases, one confusingly called "day" and the other "night". Our species' natural sleep cycle is aligned with this day-night cycle and provides a natural starting point for the definition of a day. With the exception of those constrained by occupation to a night shift, we go unconscious for eight hours mostly within the night portion of the daily cycle wherever we live. For each of us personally, our "day" begins when we wake up and ends when we go to sleep. This being largely coordinated with those near us provides a constraint on where we could reasonably put the divider between one day and the next. A universal divider between the days should occur while the bulk of us are asleep, which naturally occurs sometime at night.

As far as I could find, no culture has ever put the dividing line between days anytime between sunrise and sunset. One possible deviation is historical astronomers, who switched days at solar zenith (noon). As astronomy was a historically nighttime activity, this seems more like an exception that proves the rule and suggests how a nocturnal species might choose to delineate its days (nights?).

As a diurnal species, it seems we have settled on three possibilities: the day starts at sunset, the day starts at sunrise, or the day starts at solar nadir (midnight). The Jewish and Islamic days go from sunset to sunset. The Hindu days run from sunrise to sunrise. The European and Chinese days change over at solar nadir (midnight).

In my research, I found a lot of fuzziness and uncertainty around the sunrise and sunset definitions. It seems that it may be most natural to count the start of the day at sunrise, the end of the day at sunset, and the night being not part of any particular day. I'm guessing that in a world before electrical lighting, little happened and even less was scheduled during the night, so it didn't matter much. The need to consistently assign every particular minute of the night to one particular day or the other is a post-industrial habit that our ancestors might find a tad obsessive.

Sunrise

Everywhere on Earth gets 12 hours of daylight, on average. Everywhere on Earth, the sun rises at 6 AM, on average, ignoring timezones. That would seem to be a useful universal, except that "average" is doing a lot of work here.

Because of the tilt of the earth, the time from sunrise to sunrise is shorter in the winter and spring as the days get longer, and the time between sunrises is longer in summer and fall as the days get shorter. Depending on your latitude, this can accumulate to a very large change. At my latitude, the sun rises about an hour and a half before the average on the summer solstice and rises about an hour and a half later than the average on the winter solstice. Higher latitudes experience more extreme shifts. While the average sunrise may be 6 AM everywhere, the actual sunrise moves ahead and behind with the seasons everywhere except the equator.

Assuming that we don't want the length of the day to change every day, we have little choice but to declare the mean sunrise as the "civil sunrise" and the start of the day. But then for half the year, the day is very much flipping over during daylight, which is something we decided earlier was the only truly unnatural time to do it.

Perhaps in a post-electrical world it is not that big of a deal. We are not as driven by the sun as our ancestors. The sun may rise at 5 AM, but I'm still sleeping in. (This is not strictly true. Daylight saving time moves sunrise in this case back to 6 AM on the clock, which is a topic for another time.)

Sunset

Sunset makes even less sense to me. I am actually amazed that several cultures ended up with this convention. It drifts throughout the solar day just like sunrise, but has one additional drawback. There are about 12 hours of daylight, but humans sleep only about 8 hours. We put as many of those sleeping hours during the night, but that still leaves 4 night hours that we are typically awake for. With these extra hours, we tend to stay up late rather than get up early. This effect is so strong, it must be for biological reasons. As much as we may talk about night owls vs morning larks, going to sleep at 10 PM and waking up at 6 AM is wildly more common than going to sleep at 6 PM and waking up 2 AM, even though these are symmetric sleep schedules about the nadir. From a purely objective standpoint, humans are night owls.

Because of this bias, switching over the day at sunset invariably switches the day while almost everyone is awake and in the middle of doing things year round. Perhaps this makes sense as a religious calendar. You can have fasting days that don't actually require you to go a day without food. But as a civil calendar, this is confusing.

If we all lived on the equator or if the Earth had no tilt, it could make sense to have sunrise or sunset be the dividing line between two days. In either of those cases, the sun always rises and sets at the same time. But with the current planet and population distribution upon it, this cannot be made to work.

Midnight

Our eight-hour sleeping habits do not all perfectly overlap. And as mentioned above, there is a bias toward sleeping later rather than earlier in the night. Nevertheless, the window from midnight to 4 AM is a pretty clean deadzone. From a pure perspective of inactivity, 2 AM or 3 AM may make more sense. We switch over daylight saving time at 2 AM for this reason. But there is no astronomical meaning to 2 AM or 3 AM; it's just arbitrary.

Midnight is unique in that it is the sun's nadir, the time when the sun is at its lowest point in the sky. And fortunately for the purpose of making a regular calendar, there is the same amount of time between every nadir.

Actually, that is not strictly true. A perfectly constant period between nadirs rests on three assumptions: (1) that the period of the Earth rotation is constant, (2) that the revolution of the Earth around the sun is constant, and (3) that no other process is driving the sun higher or lower in the sky. Inaccuracies in the first assumption are on the order of milliseconds and can only be detected with atomic clocks. Inaccuracies in the second assumption contribute to variations on the order of minutes. The Earth orbit is slightly elliptical, which causes it to travel faster at some times than at others. Inaccuracies in the third assumption also contribute on the order of minutes. Because the path of the sun itself gets higher and lower with the seasons, it takes different amounts of time to reach its zenith and its nadir. Overall, the last two effects combine and the length of the solar day varies up to 15 minutes above and below the mean solar day.

Conclusion

It seems that the Gregorian calendar's division of the day, at midnight, is about as good as we can make it. This puts the division in a natural spot in our biological cycle while also aligning the day with an appropriate astronomical phenomenon.

There is one rather large wrinkle in this design though. It isn't midnight at the same time everywhere on the planet, but time zones are a topic for another time.